Abstract a-type rotational transitions of molecules in the vibrational ground state of thioketene, H2C=C=S, have been measured in the millimeter wavelength region. The measurements yielded improved rotational constants: A = 286 655(82) MHz, B = 5 659.47596(72) MHz, C = 5 544.51269(72) MHz. A detailed centrifugal distortion analysis by means of Watson's S-reduced Hamiltonian led to the determination of four quartic, two sextic and two higher order distortion constants: DJ = 1.08569(4) kHz, HJK = 0.716(20) Hz, DJK = 168.269(77) kHz, HKJ = -408.7(73) Hz, D1 = -25.46(68) Hz, LKJ = 0.65(24) Hz, d2 = - 5.21(35) Hz, SKJ = -0.0533(24) Hz. Effective rotational and centrifugal distortion constants using planarity conditions were calculated. The electric dipole moment of thioketene was determined to be μ = 1.01(3) D.